As the complexity of integrated circuits continues to increase, the dimensions of the components of the integrated circuits continue to decrease. Not only do device dimensions decrease, but the dimensions of the interconnects, that is, the lines and windows used to connect devices decrease. The windows are often termed vias by those skilled in the art. It is noted that the term window is sometimes applied only to the openings to the source, gate, or drain electrodes while the term via is applied to the opening between levels in multilevel metal structures. Vias may be conveniently visualized as cylinders which are filled with a metal so that, e.g., devices located on different metallization levels can be electrically connected. As it is difficult to completely fill a small diameter cylinder with sputtered Al, a metal commonly used in integrated circuit metallizations, a heating step is frequently used to cause an overlying dielectric layer to flow into a portion of the via. The heating step creates a more easily filled conical shape. The conical shape is, of course, truncated at the bottom so that an electrical contact can be formed to the underlying conductor.
As smaller devices typically use relatively shallower junction depths than do larger devices, the use of high temperature thermal processing to taper the vias by causing a dielectric material to flow eventually becomes impossible as the via dimensions decrease. Consequently, the vias that must be filled with metal not only have a high aspect ratio, that is, a high ratio of height to width, but their walls are also substantially vertical. Aluminum is difficult to deposit uniformly in such vias and poor step coverage results for conventional deposition techniques such as sputtering. This not only leads to possible discontinuities in the metal coverage but also makes planarization of the surface, frequently required for subsequent processing such as second level metallization, very difficult.
Al metallizations suffer from several drawbacks. Al has a low electromigration resistance and is susceptible to hillock formation. Also, because of its low melting point and tendency to react with Si, severe limitations are placed on the maximum temperature that can be used in post-metallization processing.
Accordingly, alternatives to aluminum have been sought for at least portions of the metallization. One commonly contemplated alternative metal is low pressure chemical vapor deposition (LPCVD) tungsten. LPCVD tungsten is a desirable alternative because it has a conformal step coverage. LPCVD tungsten also offers advantages for use as interconnects. Besides having conformal step coverage, it has high electromigration resistance, resistance to hillock formation and high temperature stability. Although many methods of depositing LPCVD tungsten have been proposed, they are all included within two generic categories which are conveniently termed selective and blanket.
Selective deposition typically relies upon the reaction of a gas, such as tungsten hexafluoride, with substrates, such as silicon, to leave tungsten on the silicon surface. Selective deposition is also possible on metals and silicides. A carrier gas, such as hydrogen, is commonly used. Tungsten hexafluoride does not react with other materials, such as a surrounding dielectric region of silicon dioxide, and in theory, tungsten is deposited only on the silicon surface. However, selective deposition is difficult to obtain in practice in the thicknesses which are needed to fill deep vias because of the loss of selectivity. This probably arises because some tungsten hexafluoride reacts with the hydrogen carrier gas thus forming HF as well as nucleating W. HF reacts with and etches the SiO.sub.2, which is obviously an undesirable result.
With blanket deposition, tungsten is deposited over the entire surface and then etched back so that tungsten ideally remains only in the vias or as, for example, interconnects. While this process is conceptually simple, tungsten does not adhere well to silicon dioxide and practical problems arise. For example, after the deposition has been completed, the tungsten film may simply peel off the silicon dioxide which is also an obviously undesirable result.
The use of a glue layer has been proposed to overcome the adhesion problem just discussed. A glue layer is a layer of material deposited prior to the tungsten and which has good adhesion both to the underlying dielectric layer and to the tungsten. Several substances have been proposed for glue layers. For example, the use of both elemental metals, such as Ti, and metallic silicides, such as WSi.sub.2, has been proposed. See, for example, Comparison of Two Contact Plug Techniques for Use with Planarized Oxide and A Contact Filling Process with CVD-Tungsten for Multilevel Metallization Systems, Proceedings of the V-MIC Conference, pp. 403-410, and pp. 443-449, Jun. 9-10, 1986, respectively. A common feature of the approaches adopted by these references is the use of relatively thick glue layers. Thicknesses of approximately 100 nm or more were used. It should be noted that the thickness of the Ti layer in the former reference is not explicitly given. However, FIG. 3 illustrates a plug after etching has been completed. It is evident that there is severe overetching of the glue layer which would not be visible if a thin glue layer had been used.
However, the use of a thick glue layer is undesirable because during the etch back step, severe undercutting of the tungsten layer occurs if, as is often the case, the glue layer etches more rapidly than does the tungsten. The undercutting may make subsequent processing very difficult. For example, voids may be left in the oxide and in subsequent metallizations after metal deposition.
Of course, the glue layer material should be electrically conducting. Only metals or silicides have been proposed as glue layer materials because of the rapid film growth in these materials. However, the use of some metals, such as aluminum, has not been seriously considered because a thick aluminum layer may cause spiking due to its rapid diffusion into the underlying material. Other conducting compounds have not yet been proposed as glue layers.